This invention concerns an improvement in the use of polymer flooding for tertiary oil recovery.
Oil is normally recovered from underground reservoirs by a sequence of operations. A newly drilled well generally produces a limited amount of oil by release of internal pressure in the reservoir, being augmented by mechanical pumping means as this pressure becomes depleted. This so-called primary recovery, however, results in only about 15 to 20 percent of the total oil stored in the reservoir. Other techniques must therefore be used for recovery of the remaining oil still trapped within the pores of the formation.
During secondary recovery, commonly known as water flooding, water is pumped into a well adjacent the production well to release the trapped oil and drive it to the production well. Water flooding still leaves about 60 to 70 percent of the available oil in the formation, since the water's comparatively low viscosity allows it to finger through the oil and thus leave large pockets of oil untouched. In addition, surface forces tend to bind the oil to the formation and prevent its displacement. Tertiary oil recovery is therefore commonly used for release of this remaining oil.
One approach to tertiary oil recovery is polymer flooding, which employs a mobility control solution prepared by addition of certain organic polymers to at least a portion of the injected fluid to thicken the water flood. Among these polymers are the xanthan gums, hydrophilic polysaccharides produced by fermentation using bacteria of the genus Xanthomonas. The polymer may be used in the form of the fermentation broth itself, as exemplified in U.S. Pat. No. 4,119,546, or in isolated and reconstituted form, as disclosed in U.S. Pat. No. 3,305,016. Xanthan gums are particularly desirable for polymer flooding since they are good displacing agents; give useful viscosities at low concentrations (5 to 90 centipoise at 100 to 3,000 ppm); are poorly absorbed on the porous rock formations; are relatively insensitive to salts, not precipitating or losing viscosity under normal conditions; and are shear stable, thermostable and viscosity stable over a wide pH range.
An alternative means of tertiary oil recovery is surfactant flooding, in which a surfactant is incorporated into the injection solution to lower the water-oil interfacial tension and thereby emulsify and mobilize the oil so that it may be moved to the production well. In current practice, as disclosed for example in U.S. Pat. Nos. 3,811,505, 4,077,471, 4,088,189 and 4,214,999, the surfactant, either alone or in combination with xanthan polymer, is employed at a level of from about 0.05 to 10 percent by weight of the injection solution.
Attempts at improving the injectivity and mobility characteristics of xanthan mobility control solutions by use of surfactants include U.S. Pat. No. 3,853,771, which approaches the plugging problem sometimes encountered with whole fermentation broths by employing a combination of surfactant, chelant and alkali metal hydroxide to disperse the inner and outer wall layers of the microbial cells; U.S. Pat. No. 3,801,502, which discloses a method of treating diluted xanthan fermentation broth with an alcohol, ketone, phenol or non-ionic surfactant at elevated temperature to adjust the viscosity thereof; and U.S. Pat. No. 4,212,748, which discloses a tertiary oil recovery process in which a reconstituted aqueous xanthan mixture is filtered before injection, a minor amount of surfactant being added to the mixture prior to the filtration to improve its filterability.
While such techniques have contributed to the art of polymer flooding, there still exists a need for a simple and inexpensive means of improving the injectivity and mobility of xanthan mobility control solutions, especially when incorporated in high salinity or hardness brines. It is therefore the primary objective of the present invention to satisfy this need.